10 PHYSIOLOGY OF INDUCED HYPOTHERMIA 



thermic animal has not been determined. UnpubHshed observations by the authors^'- 

 indicate that they do exist but that the direction of the gradient in certain areas is 

 reversed. Observations on right heart and rectal temperatures show a slight discrep- 

 ancy of 1° C. This difference becomes greater at rectal temperatures below 20° C. 

 because the heart cools more slowly than the rectum. Hart^^ has measured the heat 

 content of small animals by a calorimetric method and in hypothermia has found 

 rectal temperatures to be lower than average body temperature. In non-chilled ani- 

 mals average body temperature is 1 to 2° C. below rectal. These few indications of 

 variable temperatures in the hypothermic and non-chilled animal are indicative of 

 the unstable background upon which any discussion of hypothermia rests. In view 

 of the influence of temperature upon enzyme activity, the dissociation of water, the 

 pH at neutrality, the isoelectric points of proteins, the balance of electrolytes be- 

 tween cells and surrounding fluid, and many other biological phenomena, the simple 

 assumption of rectal temperature as indicative of a thermal state is likely to lead to 

 unjustified conclusions. Alterations in the chemical balances of the cell that may be 

 innocuous at the cell's optimum temperature whether it be 20^ or 40" C. may prove 

 to be disastrous if the temperature be only slightly altered. Furthermore, if two 

 adjacent units adapted to operating at a 5° or 10° C. differential are as a consequence 

 of hypothermia asked to perform at identical and lower temperatures their responses 

 and those of other units may not be in adequate harmony for the best interests of 

 the total organism. 



However, most of the available data relating energy output and temperature have 

 been referred to temperatures obtained from the rectum. Consequently, discussion 

 of the finer details of temperature and metabolic activity must be ignored. In a hiber- 

 nating animal with a body temperature of approximately 4° C, the oxygen consump- 

 tion drops to 3 to 10 per cent of normal. Considerable variation exists as to the 

 degree of depression, since it is quite difficult to secure many adequate observations, 

 due to technical difficulties. Early studies upon the oxygen consumption of humans 

 (body temperature 28° C.) and dogs, rats and rabbits (as low as 19° C.) have given 

 variable results. Values ranging from 300 per cent increase to 50 per cent decrease 

 have been recorded.^*' ^■'' ^"' ^'' ^^' -- The high values have been attributed to the in- 

 creased muscular tension and shivering invoked by cold. Bigelow ct al}^ reported 

 that oxygen consumption fell consistently with reduction in body temperature (to 

 18° C.) and rose in proportion during rewarming. According to Lynn et fl/.-° the 

 reduction in oxygen consumption is 27 per cent at a rectal temperature of 30° C. 

 and also it decreases linearly with temperature. Extrapolation to zero oxygen con- 

 sumption would place the rectal temperature at 10° C. Similar relationships for the 

 rat were noted by Adolph^'* and earlier by Woodruff-' for the dog. 



An exponential decrease in oxygen consumption and carbon dioxide production 

 with decrease in the rectal temperature was reported by Velten.-^ A similar relation- 

 ship was observed by Spurr et al.^- - and could be expressed by the e(|uation 



U)g y = 0.37x - 0.6926. 

 Their data gave Qj,, values of 2.3 in good agreement with vau't lloff's law. If 

 the animals were shivering during the development of the hypothermic state, the 

 oxygen consunij^tion followed the same pattern of response to reduction in rectal 

 temperature but at a significantly higher level (fig. 2). 



